Pulsed plasma device and method for generating pulsed plasma

ABSTRACT

A device and a method for generating a truly pulsed plasma flow are disclosed. The device includes a cathode assembly comprising a cathode and a cathode holder, an anode, and two or more intermediate electrodes, the anode and the intermediate electrodes forming a plasma channel expanding toward the anode. The intermediate electrode closest to the cathode may form a plasma chamber around the cathode tip. An extension nozzle forming an extension channel having a tubular insulator along at least a portion of its interior surface is affixed to the anode end of the device. 
     During operation, a voltage is applied between the cathode and the anode and a current is passed through the cathode, the plasma, and the anode. The voltage and current profiles are selected to cause the rapid development of a plasma flow with required characteristics. A substantially uniform temperature and power density distribution of the plasma pulse is achieved in the extension nozzle. Additionally, ozone may be generated in the extension nozzle during the generation of the plasma pulse.

CLAIM OF PRIORITY

This application is a continuation of U.S. application Ser. No.11/890,938 filed on Aug. 6, 2007.

FIELD OF INVENTION

The present invention relates to plasma generating devices, and moreparticularly to plasma generating devices and methods for producingpulsed plasma for applications requiring pure plasma.

BACKGROUND

Plasma generating devices play an important role in many areas. Forexample, plasma is used in displays, such as television sets andcomputer monitors, spectrography, in spraying applications such ascoating, and in medicine.

It is well known in the art that plasma can be effectively used in themedical field for cutting, coagulation, and vaporization of tissues. Forbest results, the generated plasma has to have precise characteristics,such as velocity, temperature, energy density, etc. Preferably, plasmaused for medical applications has to be pure. In other words, it shouldcontain only particles of the ionized plasma generating gas and no otherparticles, such as materials separated from various parts of theplasma-generating device during operation.

Recently, attempts have been made to use plasma for tissue treatment andparticularly skin treatment. Plasma may have different effects when itcomes in contact with a skin surface depending on, among others, thetemperature increase that it produces on the surface of the skin. Forexample, increasing the temperature by approximately 35°-38° C. has awrinkle reducing effect. Increasing the temperature by approximately 70°C. removes the epidermis layer, which may be useful in plastic surgery.It has been recognized that a continuous plasma flow, suitable forcutting, coagulating, and evaporation of tissues, is not suitable forother types of tissue treatment in general, and for skin treatment inparticular. Instead, to avoid undesired skin damage that would resultfrom using continuous plasma flow, pulsed plasma is used. Two types ofdevice that may be used for this purpose are presently known in the art.

The device disclosed in U.S. Pat. No. 6,629,974 is an example of thefirst type. In devices of this type, plasma is generated by passingplasma generating gas, such as nitrogen, through an alternating electricfield. The alternating electric field creates a rapid motion of the freeelectrons in the gas. The rapidly moving electrons strike out otherelectrons from the gas atoms, forming what is known as an electronavalanche, which in turn creates a corona discharge. By applying theelectric field in pulses, pulsed corona discharge is generated. Amongthe advantages of this method for generating pulsed corona discharge is(1) the absence of impurities in the flow and (2) short start times thatenable generation of a truly pulsed flow. For the purposes of thisdisclosure, a truly pulsed flow refers to a flow that completely ceasesduring the off period of the pulse.

A drawback of devices and methods of the first type is that thegenerated corona discharge has a fixed maximum temperature ofapproximately 2000° C. The corona discharge formed in the device neverbecomes a high temperature plasma. To achieve the energy of 1-4 Joulesrequired for modifying collagen during skin treatment by a device ofthis type, the rate of plasma generating gas flow has to be relativelyhigh. For example, using argon in such a device requires a flow ofapproximately 20 liters/min to achieve the required energy. That flowrate is impracticable for skin treatment. When nitrogen is used forgenerating plasma, the required energy can be achieved with a flow rateof only about 5 liters/min, but even this rate will create discomfortfor a patient. Accordingly, the applications of devices of the firsttype are limited by the nature of the electrical discharge process thatis capable of producing a corona discharge.

Devices of the second type generate plasma by heating the flow of plasmagenerating gas passing through a plasma channel by an electric arc thatis established between a cathode and an anode that forms the plasmachannel. An example of a device of the second type is disclosed in U.S.Pat. No. 6,475,215. According to the disclosure of U.S. Pat. No.6,475,215, as the plasma generating gas, preferably argon, traverses theplasma channel, a pulsed DC voltage is applied between the anode and thecathode. A predetermined constant bias voltage may or may not be addedto the pulsed DC voltage. During a voltage pulse, the number of freeelectrons in the plasma generating gas increases, resulting in adecrease in the resistance of the plasma and an exponential increase ofthe electric current flowing through the plasma. During the off period,the number of free electrons in the plasma generating gas decreases,resulting in an increase in resistance of the plasma and an exponentialdecrease in the current flowing through the plasma. Although the currentis relatively low during the off period, it never completely ceases.This low current, referred to as the standby current, is undesirablebecause a truly pulsed plasma flow is not generated. During the offperiod a continuous low-power plasma flow is maintained. In essence, thedevice does not generate pulsed plasma, but rather a continuous plasmaflow with power spikes, called pulses, thus simulating pulsed plasma.Because the off-period is substantially longer then a pulse, the deviceoutputs a significant amount of energy during the off period and,therefore, it cannot be utilized effectively for applications thatrequire a truly pulsed plasma flow. For example, if the device is usedfor skin treatment, it may have to be removed from the skin surfaceafter each pulse, so that the skin is not exposed to the low powerplasma during the off period. This impairs the usability of the device.

Dropping the current flow through the plasma to zero between pulses andrestarting the device for each pulse of plasma is not practicable whenusing the device disclosed in U.S. Pat. No. 6,475,215. Restarting thedevice for each pulse would result in the rapid destruction of thecathode, as a result of passing a high current through the cathodewithout ensuring that cathode arc attachment is well controlled.

The inability of the device disclosed in U.S. Pat. No. 6,475,215, andother devices of this type presently known in the art, to generate atruly pulsed plasma flow that can be safely used on a patient is due tothe structure of the device. It takes a few milliseconds to reach aplasma flow phase after the off period. During these few millisecondsthe plasma properties are not easily controlled, and therefore it cannotbe used on a patient. Additionally, when devices of this type startupthere is some erosion of electrodes due to sputtering. This erosionresults in separated electrode materials flowing in the plasma. When acontinuous plasma flow is used, the startup impurities are a relativelyminor drawback, because the startup, and the impurities associated withit, occur only once per treatment. It is therefore possible to wait afew seconds after the startup for the electrode materials to exit thedevice before beginning actual treatment. However, waiting forimpurities to exit the device when using a pulsed plasma flow isimpractical, because the next pulse of plasma would have to be generatedbefore the waiting period is over.

When the plasma flow has been previously created it takes just a fewmicroseconds to increase or decrease the current in the plasma flow.Additionally, because there is no startup, impurities do not enter theplasma flow, and there is no stress on the cathode. However, sustainingeven a low electrical current through the plasma continuously rendersthe device suboptimal for some applications that require a truly pulsedplasma flow, as discussed above.

Difficulties in generating a truly pulsed plasma flow by the means ofheating the plasma generating gas with an electric arc are primarily dueto the nature of processes occurring on the electrodes. In general, andfor medical applications especially, it is critical to ensure operationfree from the erosion of the anode and the cathode when the currentrapidly increases. During the rapid current increase the temperature ofthe cathode may be low and not easily controlled during subsequentrepetitions of the pulse. When generating an electric arc between thecathode and the anode, the area of attachment of the arc to the cathodestrongly depends on the initial temperature of the cathode. When thecathode is cold, then the area of attachment is relatively small. Afterseveral pulses the temperature of the cathode increases, so that duringthe period of a rapid current increase the area of attachment expandsover the entire surface area of the cathode and even the cathode holder.Under these circumstances the electric potential of the cathode beginsto fluctuate and the cathode erosion begins. Furthermore, if the area ofattachment of the electric arc reaches the cathode holder it begins tomelt thus introducing undesirable impurities into the plasma flow.

A similar situation occurs on the surface of the anode. When the currentin the arc increases rapidly, the plasma flow does not have sufficienttime to reach a high temperature. As a result, the concentration ofplasma close to the anode surface is low. This leads to a drop in theelectric potential of the anode and its fluctuation which causesintensive erosion of the cathode. Fluctuations in the electricpotentials of the cathode and anode lead to an unstable and not easilycontrolled energy of the pulsed plasma flow.

For the cathode to function properly it is necessary to control theexact location and the size of the area of attachment of the electricarc to the cathode surface during the periods of rapid current increasein each pulse of plasma. For the proper function of the anode it isnecessary to establish the flow of the heated plasma at the surface ofthe anode during the rapid current increase as well as during theoperational period of the pulse.

Generating truly pulsed plasma, especially for medical applications,poses several additional problems. First, as mentioned above, plasma hasto be pure, free from any electrode materials or other impurities.Second, properties of the generated pulse of plasma have to becontrolled. Initially, by controlling the duration, voltage and currentof the pulse the energy transferred by the pulse can be controlled. Forsome applications, such as skin treatment, merely controlling the energytransferred in the pulse is not enough; the energy and temperature haveto be distributed substantially uniformly over the treated area.

Accordingly, presently there is a need for a device that overcomes thelimitations of the currently known devices by generating truly pulsedplasma with minimal amounts of impurities, and by substantiallyuniformly distributing energy transferred in each pulse over the treatedarea. Additionally, there may be applications where the deviceoptionally needs to be capable of supplying ozone to the treated surfaceand removing fluids and other extraneous matters from the treatedsurface.

SUMMARY

The pulsed plasma device of the invention as shown in the drawingscomprises a cathode assembly which includes one or more cathodes affixedin a cathode holder, an anode, and two or more intermediate electrodes.The anode and the intermediate electrodes form a plasma channel. Theintermediate electrode closest to the cathodes also forms a plasmachamber around the cathode ends closest to the anode. The plasma channelcomprises three portions: a heating portion, an expansion portion, andan anode portion. The expansion portion has two or more expansionsections. The diameter of each successive section of the expansionportion increases toward the anode. The anode portion has a diameterthat is greater than the diameter of the expansion portion closest tothe anode. The cathode holder prevents displacement of the cathodes,preferably keeping them parallel to the axis of the device thuspreventing their angular displacement.

An extension nozzle is affixed at the anode end of the device. Theextension nozzle forms an extension channel connected to the plasmachannel. A tubular insulator element covers a longitudinal portion ofthe inside surface of the extension channel. Additionally, in someembodiments, the extension nozzle has one or more oxygen carrying gasinlets.

During operation truly pulsed plasma is generated by the device. Foreach pulse, the plasma passes through three stages: a spark discharge, aglow discharge, and an arc discharge. In an exemplary embodiment, thespark discharge is created by applying a high frequency, high amplitudevoltage wave between the cathodes and the anode. After the sparkdischarge is created between the cathodes and the anode, a preferablytransient voltage is applied between the cathodes and the anode and acurrent is passed through the cathodes, the plasma generating gas, andthe anode, which results in generation of the glow discharge. At the endof the glow discharge, when the cathode ends become sufficiently heated,the voltage between the cathodes and the anode drops down marking thebeginning of the cathode thermionic electron emission and the beginningof the arc discharge phase. Once the arc discharge phase begins theplasma is attached to all cathodes in the assembly. The current islowered to decrease the area of plasma attachment to a single cathode.Then after the lowered current is maintained for a period of time, thecurrent is increased to the operational level. After the predeterminedduration of the pulse, both the voltage and the current are set to zerofor the duration off period. This process is repeated for every pulse.

For medical applications, it is critical that there be no impurities inthe plasma. Sputtering from the surface of the cathode holder iseliminated by utilizing a cathode assembly with multiple cathodes andgenerating pulsed plasma having a controlled area of plasma attachment.

During operation, the plasma flow exiting the anode has an essentiallyparabolic temperature and energy density distribution. The extensionnozzle transforms the temperature and energy distribution to a moreuniform distribution that is more suitable for contact with a patient.The thermal insulator located in the extension channel is made of anon-metal material with a low thermal conductivity. When the plasmaflows through this thermal insulator, the colder layers of plasma areheated without transferring heat to the elements forming the channel.

Additionally, in embodiments having inlet passages to the nozzle, whilethe plasma flow traverses the extension channel, it sucks oxygencarrying gas, such as air, into the flow. Under the influence of thehigh temperature of plasma in the extension channel and the radiationemanating from the plasma channel, ozone is formed in the extensionchannel. Molecules of ozone, which may have beneficial effects, exit thedevice together with the plasma and come in contact with the treatedskin.

In one embodiment, a device for generating pulses of plasma comprising:an anode; a cathode assembly comprising (i) one or more cathodes, and(ii) a cathode holder; a plasma channel, extending longitudinallybetween said cathode and through said anode, and having an outletopening at the anode end, a part of said plasma channel being formed bytwo or more intermediate electrodes electrically insulated from eachother and the anode, the plasma channel comprising a heating portionclosest to the cathode, an anode portion, and an expansion portionbetween the heating portion and the anode portion, the expansion portionhaving two or more sections with diameters of the sections increasingtoward the anode, wherein the minimum number of sections of the heatingportion depends on the ratio of the diameter of the plasma channel inthe anode portion and the diameter of the plasma channel in the heatingportion; a plasma chamber formed by one of the intermediate electrodes,the plasma chamber connected to the cathode end of the plasma channel;and an extension nozzle forming an extension channel connected to theanode end of the plasma channel is disclosed.

Also, a method of treating tissue with pulses of plasma comprising: foreach pulse, generating a plasma flow; expanding the plasma flow to apredetermined cross-section; modifying the distribution of thermalenergy of the expanded plasma flow so the distribution is substantiallyuniform in the cross-section; applying the plasma flow to the treatedskin; and ceasing the plasma flow is disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a system for generating of pulsed plasma comprising aconsole and a hand piece;

FIG. 2A illustrates a cross sectional longitudinal view of an embodimentof the device;

FIG. 2B illustrates a cross sectional longitudinal view, transversely tothe view illustrated in FIG. 2A;

FIG. 2C illustrates a cross sectional longitudinal view of an embodimentof the device configured to generate ozone;

FIG. 3 illustrates a coolant divider;

FIG. 4 illustrates the preferred configuration of the cathode assembly;

FIG. 5 illustrates the geometry of the plasma chamber;

FIG. 6 illustrates a heating portion, an expansion portion, and an anodeportion of the plasma channel and sections of the expansion portion;

FIG. 7A illustrates an embodiment of the device comprising a suctionmodule in which extraneous matter is collected in the console;

FIG. 7B illustrates an embodiment of the device comprising a suctionmodule in which extraneous matter is collected in an outside container;

FIG. 7C illustrates a cross sectional view of an embodiment of thedevice comprising an extension nozzle with oxygen carrying gas inletsand a suction module;

FIG. 7D illustrates a cross sectional view of an embodiment of thedevice comprising an extension nozzle without oxygen carrying gas inletsand a suction module;

FIG. 8A illustrates the voltage applied between the cathode and theanode during generation of a plasma pulse;

FIG. 8B illustrates the current passed through the cathode, the plasma,and the anode during generation of the plasma pulse;

FIG. 8C illustrates the temperature profile of the plasma in the heatingportion of the plasma channel;

FIG. 8D illustrates the power density profile of the plasma in theheating portion of the plasma channel;

FIG. 9 illustrates (1) the widening of the plasma channel that does notresult in the full expansion of plasma during the pulse and (2) anelectric arc established between the plasma and an intermediateelectrode;

FIG. 10A illustrates a substantially parabolic temperature and powerdensity distribution of the plasma flow exiting the anode;

FIG. 10B illustrates the effect on a tissue when treated with the plasmaflow having the temperature and power density distribution illustratedin FIG. 10A;

FIG. 11A illustrates a substantially uniform temperature and powerdensity distribution of the plasma flow exiting the extension nozzle;

FIG. 11B illustrates the effect on a tissue when treated with the plasmaflow having the temperature and power density distribution illustratedin FIG. 11A;

FIG. 12A illustrates an extension nozzle having oxygen carrying gasinlets of a relatively small diameter;

FIG. 12B illustrates an extension nozzle having no oxygen carrying gasinlets; and

FIG. 13 illustrates a spectral distribution of light exiting the device.

DESCRIPTION OF EMBODIMENTS

Referring to FIG. 1, a system for generating pulsed plasma generallycomprises a console 100 and a hand piece 200. Hand piece 200 issometimes referred to herein as the device. Console 100 supplieselectricity, plasma generating gas, preferably argon, cooling agents,such as water, and/or oxygen carrying gas, such as air, etc. to handpiece 200. Additionally, console 100 may contain one or more pumps thatmay be used to remove extraneous matter from the treated surface withthe device 200 via one or more suction channels. Console 100 has controlcircuitry for operating hand piece 200 and a user interface comprised ofa display and controls, which are generally known in the art. Anoperator, such as a trained medical professional, programs the mode ofoperation of the device with the controls of console 100 in accordancewith parameters for a given medical procedure, and then uses hand piece200 to perform the given procedure. Although the description ofembodiments of the present disclosure relates to the medical field, itis understood that other embodiments of the device may be used for otherapplications unrelated to medicine.

FIG. 2A shows a longitudinal cross section of one embodiment of thedevice. In this embodiment, casing 9 forms the outside of the device.Preferably, device 200 is cylindrical and all elements are annular andare arranged coaxially with respect to the axis of the device 50. Insome embodiments, however, the device may not be cylindrical and adifferent internal or external geometry may be used. The embodiment ofthe device shown in FIG. 2A comprises anode 1 and one or more cathodes 5a, 5 b, 5 c, preferably made of tungsten containing lanthanum, arrangedin a cathode holder 6. The cathode holder 6 prevents undesired angulardisplacement of the cathode from the axis of the device 50. Cathodeholder 6 also holds a portion of conductor 7, which is preferably a rodmade of metal with high electric conductivity. Crimping of thecomponents of the cathode assembly together during manufacture is onemeans for preventing displacement of the cathode along the axis 50. Inthe preferred embodiment, the entire cathode assembly, comprised ofcathodes 5 a, 5 b, 5 c, cathode holder 6, and electric conductor 7, istightly fitted inside device 200 to prevent any movement of cathodes 5a, 5 b, 5 c.

Cathode insulator 11 surrounds longitudinal portions of cathode 5 a, 5b, 5 c. Cathode insulator 11 extends from the surface of the cathodeholder 74 closest to the anode on one end to a point part way along thecathodes. Cathode insulator 11 is made of a material that provides boththermal and electrical insulation of cathode 5 a, 5 b, 5 c. Electricalconductor 7 is used to supply electric potential to cathodes 5 a, 5 b, 5c. Cathodes 5 a, 5 b, 5 c are electrically connected and always have thesame electric potential. Insulator element 8 surrounds conductor 7 and aportion of cathode holder 6 as shown in FIG. 2A. Insulator element 8provides electrical insulation of conductor 7, and is preferably made ofvinyl. In operation, a passage 72 enables the flow of the plasmagenerating gas from console 100 along insulator 8. The gas then flowsalong space 54 between insulator 8 and water divider 10. Then, the gasflows through groove 56 in cathode holder 6, and then along cathodes 5a, 5 b, 5 c, inside the cathode insulator 11.

In the preferred embodiment the cathode assembly may be the one shown inFIG. 4. Briefly, the cathode assembly comprises two or more cathodes. Inthe preferred embodiment shown in FIG. 4, the cathode assembly comprisesthree cathodes 5 a, 5 b, 5 c. Preferably, the diameter of each cathodeis 0.5 mm. The combined diameter of the three cathodes in the preferredembodiment is approximately 1 mm. At least one of the cathodes has adifferent length compared to other cathodes of the cathode assembly.Preferably, however, each cathode has a different length from all othercathodes in the assembly. The difference in length between two cathodesthat have the closest lengths preferably equals to the cathode diameter,which is 0.5 mm in the preferred embodiment. The difference in lengthamong the cathodes creates a natural surface imperfection to which theelectric arc tends to attach.

In an alternative embodiment, a cathode assembly with a single cathodemay be used, but such an embodiment is limited to certain applicationsthat require a limited number of pulses. Cleaning of substrate surfaceswith plasma pulses in microchip manufacturing is one such application.The embodiment of the device with the cathode assembly with a singlecathode is suited to generate at most 300-500 pulses. After about 500pulses, the temperature in the entire cathode body increases. This leadsto an expansion of the area of contact between the plasma and thecathode surface when the arc is started. As a result, the plasma comesin contact with the cathode holder. Once the cathode holder begins tomelt, it damages the cathode at the point where the cathode and thecathode holder are connected, creating an imperfection in that spot onthe cathode. Once the imperfection is created the electric arc tends toattach to that imperfection, instead of the tip of the cathode, whichdisrupts the normal process of pulsed plasma generation and results inoperational instability of the device. After the device cools down toroom temperature, it is capable of generating another train of 300-500pulses. Therefore, this alternative embodiment may be used forapplications that require a limited number of pulses not exceedingapproximately 500. The maximum number of pulses may be increased, butonly insignificantly, by increasing the length of the cathode, therebydistancing the end of the cathode closest to the anode from the cathodeholder.

Alternatively, to overcome the problem of operational instability of theembodiments of the device using a single cathode after a few hundredpulses, the cathode may be “trained” in the continuous plasma modebefore switching to the pulsed plasma. Cathode training refers to theoperation of the device in the continuous plasma mode, passing a high DCcurrent through the cathode. Because initially, due to the geometry ofthe cathode and, in some embodiments the geometry of the plasma chamber,the electric arc attaches to the tip of the cathode, passing DC currentthrough the tip of the cathode for a sufficiently long time creates asurface imperfection right at the tip of the cathode. After the cathodehas been “trained,” when the device is switched to the pulsed plasmamode, the electric arc attaches to the imperfection at the tip of thecathode that was created by the “training.”

During operation, the plasma generating gas flows from console 100 todevice 200. The plasma generating gas enters the device through passage72. After the plasma generating gas passes cathode insulator 11, itpasses through a plasma channel 62. This direction is referred to as thedirection of the plasma flow. In the embodiments comprising a plasmachamber, the plasma generating gas passes plasma chamber 26 before itenters plasma channel 62. Plasma channel 62 is formed by anode 1 and twoor more intermediate electrodes. The end of the plasma channel furthestfrom the cathodes is referred to as the anode end of the plasma channel;similarly the end of the plasma channel furthest from the anode isreferred to as the cathode end of the plasma channel. Plasma channel 62has an outlet at the anode end. In the embodiment shown in FIG. 2A,plasma channel 62 is formed by anode 1 and intermediate electrodes 2, 3,and 4. Intermediate electrode 4 also forms plasma chamber 26. In otherembodiments, the electrode that forms plasma chamber 26 does not form aportion of plasma channel 62. Insulators 14, 13, and 12 provideelectrical insulation between pairs of adjacent electrodes. Insulator 14provides electrical insulation between the anode 1 and intermediateelectrode 2; insulator 13 provides electrical insulation betweenintermediate electrodes 2 and 3; and insulator 12 provides electricalinsulation between intermediate electrodes 3 and 4. Principles of plasmageneration using a multi-electrode system are well known in the art.

FIG. 2B shows a longitudinal cross section transverse to the crosssection shown in FIG. 2A of the embodiment of device 200 shown inFIG. 1. FIG. 2B shows a cooling system comprising inlet 76, forwardcoolant channel 78, circular cooling channel 42 (shown in FIG. 2A),reverse coolant channel 79, and outlet 77. A coolant, preferably water,enters the device through inlet 76. The coolant then traverses forwardcoolant channel 78 along plasma channel 62 in the direction of theplasma flow, from cathodes 5 a, 5 b, 5 c to anode 1. In the area ofanode 1, forward coolant channel 78 connects to circular channel 42,shown in FIG. 2A. The coolant flows along circular channel 42 aroundanode 1. On the diametrically opposite side of the device, circularchannel 42 connects to reverse coolant channel 79. The coolant flowsalong reverse coolant channel 79 in the direction opposite to the plasmaflow, from anode 1 to cathodes 5 a, 5 b, 5 c, and then exits the devicethrough outlet 77.

FIG. 3 shows a coolant divider 10, which together with other elementsforms forward coolant channel 78, circular channel 42, and reversecoolant channel 79. Some embodiments of the device may comprise multiplecooling systems. Such embodiments also comprise multiple coolantdividers that would form the respective channels of other coolingsystems.

If device 200 is subject to size constraints, such as for key holesurgeries, a plasma chamber may be used. FIG. 5 shows plasma chamber 26in greater detail. The geometry of the plasma chamber 26 is critical toproper functionality of the device. Cathodes 5 a, 5 b, 5 c tend to emitelectrons from imperfections in their surfaces, for example their edges68 a, 68 b, 68 c, respectively. For proper operation, at the start ofeach pulse a spark discharge has to be established between one ofcathode edges 68 a, 68 b, 68 c and a point on the inside surface ofplasma channel 62. To accomplish this, the following condition has to besatisfied. The distance between any of cathode edges 68 a, 68 h, 68 cand a point on the inside surface of the plasma channel, for examplepoint 64, must be less than or equal to the distance between any ofcathode edges 68 a, 68 b, 68 c and any other surface onto which anelectric spark may terminate, such as the inside surface of plasmachamber 26 and the edge of the cathode insulator closest to anode 66. Ifthe geometry of plasma chamber 26 and cathodes 5 a, 5 b, 5 c shown inFIG. 5 is used, during the startup of the device, an electric sparkoccurs between cathodes 5 a, 5 b, 5 c and a point on the inside surfaceof plasma channel 62, for example point 64. This ensures properoperation of the plasma generating process. This geometry of plasmachamber 26 also results in other benefits, such as decreasing the timerequired to achieve the arc discharge phase, which is critical forpulsed plasma, as described below. If device 200 is not subject to sizeconstraints, which is the case for most skin treatment applications, theplasma chamber is optional.

FIG. 6 shows the structure of plasma channel 62. Plasma channel 62comprises heating portion 84, expansion portion 82, and anode portion83. Expansion portion 82 and anode portion 83 are used to expand theplasma flow to the required cross-sectional area for a givenapplication. Expansion portion 82 comprises one or more expansionsections. In the embodiment shown in FIG. 6, expansion portion 82comprises expansion sections 86, 88, and 90.

In the preferred embodiment, heating portion 84 is formed by two to fiveintermediate electrodes. In alternative embodiments, heating portion 84may be formed by a single intermediate electrode or by six or moreintermediate electrodes. The diameter of heating portion 84, d_(hp), ispreferably in the range of 0.5-1.5 mm. The length of each electrodeforming heating portion 84, l_(e) _(—) _(hp), depends on d_(hp) and ispreferably in the range of d_(hp) to 2×d_(hp). The length of the entireheating portion depends on the flow rate of the plasma generating gas—alonger heating portion is required to heat a greater plasma generatinggas flow. For a typical flow rate of plasma generating gas, which is inthe range of 1 to 2 l/min, the heating portion is formed by at leastthree intermediate electrodes. The length of the entire heating portion,l_(hp), may be approximated by multiplying the number of intermediateelectrodes that are required to form the heating portion by the lengthof such an intermediate electrode, l_(e) _(—) _(hp).

The number of sections in expansion portion 82 depends on the diameterof the heating portion 84 and the diameter of anode portion 83, and isgoverned by the following relationship:

${N_{S} \geq {\frac{d_{A} - d_{h\; p}}{c} - 1}},{where}$

N_(S) is a number of sections in the expansion portion of the plasmachannel,

d_(A) is the diameter of the anode portion of the plasma channel inmillimeters,

d_(hp) is a diameter of the heating portion of the plasma channel inmillimeters, and

c, for this and other equations, is a constant in the rage between0.2-0.6 mm, preferably 0.4 mm. Although c may be chosen to be less than0.2, as will be shown below, choosing such value of c results in animpracticable length of device 200.

For the purposes of this disclosure, the sections of expansion portion82 are counted from cathodes 5 a, 5 b, 5 c toward the anode 1. So thatsection 86 is section no. 1; section 88 is section no. 2; section 90 issection no. 3, etc. If a particular embodiment has more than threesections they are counted in this manner as well. The dimensions of thesections of expansion portion 82 are preferably governed by thefollowing relationships:

d_(n) is preferably d_(n-1)+c,

l_(n) is preferably between d_(n) and 2×d_(n), where

n is the section number of a given section,

d_(n) is the diameter of the nth section, and

l_(n) is the length of the nth section.

For determining the diameter of section no. 1, section 86 in FIG. 5, thevalue of d₀, required to compute d₁, is set to the diameter of theheating portion, d_(hp).

The dimensions of anode portion 83 are preferably governed by thefollowing relationships:

d_(A) is preferably d_(Z)+c,

l_(A) is preferably between 2×d_(A) and 5×d_(A), where

d_(A) is a diameter of the anode portion,

l_(A) is a length of the anode portion, and

z is the number of expansion section of expansion portion 82 closest tothe anode. In FIG. 6, z is 3, which is the number of expansion section90.

In the preferred embodiment, cross-sections of the plasma channeltransverse to the longitudinal direction of the plasma channel arecircles. In other embodiments, however, cross-sections may have adifferent geometric shape.

In some embodiments of the device, each section of the expansion portionis formed by a separate intermediate electrode. In other embodiments ofthe device, a single intermediate electrode may form portions of two ormore adjacent sections. In yet some other embodiments, some intermediateelectrodes may form a portion of a section, or an entire section, of theexpansion portion, and other intermediate electrodes may form onlyportions of two or more adjacent sections. In the embodiment shown inFIG. 2A, intermediate electrode 3 forms section 86 (and heating portion84), and intermediate electrode 2 forms sections 88 and 90.

Device 200 includes an extension nozzle. For example, turning back toFIG. 2A, extension nozzle 15 is affixed to the anode end of the device.Extension nozzle 15 has extension channel 18, which extends the plasmachannel. A portion of the extension channel 18 is formed by tubularinsulator element 17 made of a ceramic material or quartz. Tubularinsulator element 17 prevents the termination of an electric dischargein extension nozzle 15. That, in turn, prevents electrode materials fromseparating from extension nozzle 15 and entering the plasma flow duringoperation of device 200. This ensures the purity of the plasma exitingan extension channel from outlet 55. The diameter of the portion ofextension channel 18 formed by tubular insulator element 17 ispreferably in the range of 1.0-1.3 times the diameter of anode portion83 of plasma channel 62. The length of extension channel 18 ispreferably 2-3 times its diameter. In those embodiments that areconfigured for generation of ozone, extension nozzle 15 also has one ormore oxygen carrying gas inlets 16 as shown in FIG. 2C. Inlets 16 areused for supplying oxygen carrying gas, preferably air, to extensionchannel 18, which may be used for production of ozone, as describedbelow.

The computations of dimensions of different elements in the preferredembodiment of the device are illustrated by the following example.Assume the heating portion has a diameter of 1.0 mm and a length of 1.5mm (which are governed largely by the flow rate of the plasma generatinggas) and the desired diameter of the plasma flow exiting the device fromthe outlet of extension channel 55 is 4.8 mm. The diameter of theextension channel would be 4.8 mm and its length may be set to anylength in the range of 2-3 times its diameter, for example 14.0 mm. Thediameter of the extension channel should be 1.0-1.3 times the diameterof the anode portion of the plasma channel, and is preferably between 6mm and 12 mm. In this example, if the diameter of the extension channelis 1.2 times the diameter of the anode portion of the plasma channel,the diameter of the anode would be 4.0 mm. The length of the anodeportion may be any length between 2 times its diameter and 5 times itsdiameter. In this example, when the length of the anode is set to 3times its diameter, the length would be 12.0 mm. The expansion portionexpands the diameter of the plasma channel from the diameter of heatingportion 84, which in this example is 1.0 mm, to the diameter of theanode portion of the plasma channel, which is 4.0 mm. Accordingly, inthis example the expansion portion expands the diameter of the plasmachannel by 3.0 mm. This expansion may be accomplished in a number ofways. For example, diameters of each section of the expansion portionmay increase by the maximum c of 0.6 mm. In this case, the number ofsections in the expansion portion, N_(s) is 4. The diameters of thesesections are as follows: 1.6 mm, 2.2 mm, 2.8 mm, and 3.4 mm. The lengthsof the sections may be set to any values between one and two times thediameter. Accordingly, the length of the sections may be: 2.0 mm, 3.0mm, 4.0 mm, and 5.0 mm, respectively. If the diameter increase of eachsection is chosen to be less than 0.6 mm, then more sections in theexpansion portion are needed.

Alternatively, c may be set to the preferred value of 0.4 mm. In thiscase, the number of sections in the expansion portion, N_(s), is 7. Thediameters of these sections are as follows: 1.4 mm, 1.8 mm, 2.2 mm, 2.6mm, 3.0 mm, 3.4 mm, and 3.8 mm. The lengths of the sections may be 3.5mm, 4.5 mm, 5.5 mm, 6.5 mm, 7.5 mm, 8.5 mm, and 9.5 mm, respectively.Note that in this example, the expansion between the expansion sectionclosest to the anode and the anode is only 0.2 mm, as opposed to 0.4 mm.This does not impair the functionality of the device.

It is further possible to have a different expansion between differentpairs of sections of the expansion portion. For example, the expansionfrom the heating portion to the first section may be 0.4 mm, and theexpansion between other expansion sections and the anode may be 0.5 mm.

The above discussion presumes that the intermediate electrodes, theanode, and the extension nozzle are annular, thus making the portionsand sections of the plasma channel and the extension channelcylindrical. As mentioned above, in some embodiments other geometry ofthe device may be used. In those embodiments, the diameter of the crosssection transverse to the longitudinal direction of the plasma channel,which, for the purposes of this disclosure is the largest distancebetween any two points of a shape, remains the critical dimension forpurposes of the foregoing calculations.

As mentioned above, when plasma is used to perform medical procedures,extraneous matter, that may attenuate the effect of the plasma, may becreated. For example, during a medical procedure, particles or piecesmay separate from the treated tissue, and then interfere with, or eveninterrupt, the plasma flow to the target area of the treated tissue.Also, during certain medical procedures bodily fluids, such as blood,lymph, etc. may enter onto the surface of the treated area. Those fluidsmay also interfere with the effectiveness of plasma. Some embodiments ofthe device include a suction module for removing such extraneous matterfrom the treated surface during medical procedures. FIG. 7A shows anembodiment of the device with a suction module. In this embodiment,outer casing 92 encloses the device shown in FIGS. 2A-C. Outer casing 92has one or more suction channels 94, 96. A pump operating inside console100 sucks the extraneous matter from the treated tissue. The extraneousmatter flows along channels 94 and 96, and then to console 100, where itis accumulated in a collection unit (not shown in FIG. 7A). FIG. 7Bshows a different embodiment of the device with a suction module. Thisembodiment is similar to the embodiment shown in FIG. 7A except thatchannels 94 and 96 do not extend along the partial length of the deviceand are connected with outlet 98. Note that in embodiments that includeboth a suction module and an extension nozzle with oxygen carrying gasinlets, the inlets extend through casing 92 as illustrated in FIG. 7Cthat shows cross section A-A in FIG. 7A. FIG. 7D shows cross section A-Ain embodiments comprising an extension nozzle without oxygen carryinggas inlets.

In the preferred embodiment the device generates truly pulsed plasma.After each plasma pulse, during the off period, the flow of plasmacompletely ceases until the next pulse. Between the pulses, during theoff period, the electric current does not flow between the cathode andthe anode and no plasma is generated.

Console 100 has one or more electronic circuits for controlling thecurrent through the plasma channel and applying the voltage between thecathode and the anode. These circuits are used for generation of eachplasma pulse. As a brief overview, the process of plasma generationincludes three phases: a spark discharge, a glow discharge, and an arcdischarge. During the arc discharge phase, an electric arc of apredetermined current that is established between one of the cathodesand the anode, heats the plasma generating gas flowing in plasma channel62 and forms plasma. Generation of each plasma pulse requires the plasmagenerating gas to go through all three phases. Prior to generation of apulse, the resistance of the plasma generating gas is close to infinity.A small number of free electrons are present in the plasma generatinggas due to ionization of atoms by cosmic rays. The plasma formationprocess is controlled by (1) applying the voltage applied between thecathode and the anode as shown in FIG. 8A and (2) controlling thecurrent passing through the plasma as shown in FIG. 8B.

The method of operating the device depends on the structure of thecathode assembly and may be modified depending on the configuration ofthe device and a particular application for which it is used. In thepreferred embodiment of the device having a cathode assembly comprisingmultiple cathodes, a method of operation specifically adapted for thecathode assembly shown in FIG. 4 is used. Briefly, to create a sparkdischarge a high amplitude, high frequency voltage wave is appliedbetween anode 1 and cathodes 5 a, 5 b, 5 c. This wave increases thenumber of free electrons in plasma channel 62, between cathodes 5 a, 5b, 5 c and anode 1. The frequency, duration, and amplitude of the wavedepends on the geometry of the device. Once a sufficient number of freeelectrons has been formed, a DC voltage is applied between anode 1 andcathodes 5 a, 5 b, 5 c and a DC current is passed through the cathodes,plasma generating gas, and the anode, forming a spark discharge betweencathodes 5 a, 5 b, 5 c and anode 1.

Thereafter the resistance of the plasma generating gas drops and theglow discharge phase begins. During the glow discharge phase, positivelycharged ions, formed as a result of ionization, are attracted to thecathode under the influence of the electric field created by the voltagebetween cathodes 5 a, 5 b, 5 c and anode 1. As cathodes 5 a, 5 b, 5 care being bombarded with ions, the temperature of the cathode endsclosest to anode 1 increases. Once the temperature increases to thetemperature of thermionic electron emission, the arc discharge phasebegins. As mentioned above, the surface area and volume of plasmachamber 26 provide a large number of ions, which shortens the time ofthe glow discharge phase.

Once the arc discharge begins, the plasma is attached to all cathodes inthe assembly. The current passing through the plasma is then dropped,causing the area of attachment to decrease to almost the minimum area ofattachment capable of sustaining the arc discharge. This minimal area isreferred to as the spot attachment area. Because the area of plasmaattachment is small, the attachment occurs only at a single cathode.Therefore, the current required to sustain the arc discharge, which isproportional to the cathodes diameter, is relatively low. After thecurrent has been reduced and kept at the level for a period of time, itis increased rapidly to the operational level of a pulse. The area ofattachment of plasma increases insignificantly, and only a singlecathode continues to emit electrons for the rest of the pulse.Decreasing the area of plasma attachment, and then maintaining the smallarea, so that only a single cathode emits electrons from a controlledarea is critical to the operation of the device.

As mentioned above, in different embodiments, variations of this methodof operation may be used. For example, in the alternative embodimentwith a single cathode, the area of attachment may only be controlledwith the length of the cathode and tapering of the cathode end orcathode training. In those embodiments, the current is increased to theoperational level as soon as the arc discharge phase is reached.

The geometry of elements in the disclosed embodiments and the shape andsynchronization of the voltage and current pulses ensure that thecathodes (or the cathode, depending on the embodiment) are not subjectedto the stress of high current being passed through it when there is nothermionic electron emission sufficient to support the current. That inturn ensures that the device may be started thousands or even tens ofthousands times with the same cathode assembly.

The relationships governing the dimensions of different sections in theexpansion portion allow the plasma to expand rapidly, during theoperational period of the pulse, which is critical for generating apulse of plasma with required characteristics. It has beenexperimentally found that a single increase in the diameter of theplasma channel by more than 0.6 mm results in incomplete plasma flowexpansion, or even no expansion at all, during the operational period ofthe pulse. In other words, if the diameter of an nth section of theexpansion portion is increased by more than 0.6 mm compared to thediameter of the (n−1)th section, the plasma flow does not expand to thediameter of the nth section, and the plasma flow is restricted to aparticular cross-section that is smaller than the cross section of thenth section while it traverses the remaining downstream portion of theplasma channel and the extension channel. FIG. 9 illustrates thisconcept. In FIG. 9, there is no expansion portion 82. The heatingportion 84 a transitions into anode portion 83 a. The diameter of theanode portion 83 a exceeds the diameter of the heating portion 84 a bymore than 0.6 mm. The plasma flowing in plasma channel 62 a does notexpand, or expands insufficiently, during the operational period of thepulse, when the plasma enters the anode portion 83 a. A very similarsituation occurs when there is an expansion portion, but the differencein the diameter between adjacent sections exceeds 0.6 mm. However, whenthe dimensions of the expansion sections are within the ranges governedby the relationships set forth above, the plasma flow expands to theentire cross-section at the anode end of each section, so that thediameter of the plasma flow equals the diameter of the extension channelat the outlet 55. Note that for longer pulses or continuous plasma flow,increases of 0.6 mm and above may result in partially expanded plasmaflow. Essentially, for brief pulses required for such applications asskin treatment, a single diameter increase has to be less than or equalto 0.6 mm so that the plasma flow fully expands to the increaseddiameter in each section.

Another problem presented by a single diameter increase of more than 0.6mm is the potential for the formation of an electric arc between theplasma flow and a wall of the anode, if the plasma flow is separatedfrom the wall. This is also shown in FIG. 9. FIG. 9 shows electric arc171 formed between the plasma flow and the wall of the anode 1. Such anelectric arc would introduce electrode materials into the plasma flowand would make the plasma impure. This process of gradual widening ofthe plasma flow plays a major role in generating a truly pulsed plasmaflow, when the current in the arc rapidly increases while the plasma inthe flow has not been heated sufficiently.

Increasing the diameter of the plasma channel by less than 0.2 mmresults in neither impurities nor insufficient expansion of plasma.However, expansion of less than 0.2 mm is also undesirable. Inparticular, a device with expansions of less than 0.2 mm would require agreater number of expansion sections. Each expansion section has itsminimal length requirements, so having a greater number of expansionsections means having a longer and less convenient device. Additionally,aside from mere inconvenience, an increased number of expansion sectionsrequires more energy and therefore greater power for heating the plasmaflow that traverses a plasma channel the length of which is increaseddue to the increased number of the expansion sections. Accordingly,although the device would function properly even with increases ofsection diameters of less than 0.2 mm, it is preferable that eachexpansion is within the range of 0.2-0.6 mm.

As the plasma expands in expansion portion 82, some of its propertieschange. During the operational period of the pulse, the heating portionis characterized by a power density in the range of 0.3-5 kW/mm³, asshown in FIG. 8D. The average velocity of the plasma flow in the heatingportion is preferably less than or equal to 500 m/s. The averagetemperature of the plasma is 8-18° kK, preferably 10-16° kK, as shown inFIG. 8C. The electric field in the heating portion is preferably in therange of 2-25 V/mm.

The expansion portion is characterized by a power density of less than0.3 kW/mm³. The average temperature of the plasma in the expansionportion preferably remains in the range of 8-18° kK. The electric fieldin the expansion portion of the plasma channel is preferably within arange of 1-5 V/mm.

After the plasma flow expands in expansion portion of the plasma channel82, it reaches extension nozzle 15. Extension nozzle 15 has a dualeffect on plasma flow: first it changes the temperature and energydistribution of the plasma flow to make it suitable for a particularapplication, such as tissue treatment and second, it may create ozoneand nitric oxide in the plasma flow.

The first effect of extension nozzle 15 on the plasma flow is changingthe temperature and energy distribution of the plasma flow. During thearc discharge phase, the electric arc between the cathode and the anodeheats the plasma in plasma channel 62. Only a small fraction of theplasma forms the center of the plasma flow where the temperature ishigh. The remaining plasma flows along the periphery of the plasmachannel at a distance from the electric arc, and therefore has asubstantially lower temperature. The plasma flowing along the peripheryof the plasma channel cannot be heated to the same temperature as theplasma flowing in the center because the intermediate electrodes and theanode forming the plasma channel are made of metals with a high thermalconductivity. Accordingly, the heat transferred from the plasma flowingin the center to the plasma flowing along the periphery is transferredto the intermediate electrodes and the anode and is not retained by theplasma flowing along the periphery. When the plasma reaches the anodeend of plasma channel 62, it has a substantially parabolic temperaturedistribution as shown in FIG. 10A. As illustrated in FIG. 10A, thetemperature of the plasma in the center of the plasma flow issubstantially higher than the temperature at the periphery of the plasmaflow. Similarly the energy density of the plasma, which is proportionalto the temperature, is substantially higher in the center of the plasmaflow than at the periphery.

Such temperature and energy density distribution of the plasma flow isnot suitable for some applications, such as skin treatment. When a pulseof plasma flow with such temperature and energy density distributioncomes in contact with the skin of a patient, a small area of the skinabsorbs most of the energy in the plasma flow pulse, and a much largerarea absorbs the remainder of the energy. FIG. 10B shows circular area190 of the skin having the diameter of the plasma channel in the anodeportion. If a pulse of plasma flow as shown in FIG. 10A comes in contactwith area 190, an approximately 20% fraction 192 of area 190 absorbsapproximately 80% of the energy stored in the plasma pulse. Theremaining 80% fraction 194 of area 190 absorbs only about 20% of theenergy stored in the plasma pulse.

The extension nozzle changes the temperature and energy densitydistribution to a substantially uniform one as illustrated in FIG. 11A.In other words, the temperature and energy density of the plasma flowthat exits the extension nozzle from outlet 55 is approximately the samein the entire cross section of the flow. FIG. 11B illustrates an area ofskin treated with a plasma flow having the distribution shown in FIG.11A. As shown in FIG. 11B, when a pulse of plasma flow comes in contactwith the skin, the entire area is affected by the pulse substantiallyuniformly, and there are no spots that receive substantially more orsubstantially less energy. In the preferred embodiment, the geometry ofthe elements in the device as well as operational parameters (i.e.plasma generating gas flow rate, magnitude of the current, etc.) areselected in such a way that the energy density of the plasma applied tothe treated tissues is 5-500 J/cm². In other embodiments, other energydensity may be achieved.

As mentioned above, when the plasma exits the anode and enters theextension channel, its temperature and energy density have a parabolicdistribution as shown in FIG. 10A. One of the main reasons that theplasma flow does not achieve uniform temperature in the plasma channelis that intermediate electrodes and the anode are made of metal, such ascopper, having a high thermal conductivity. Due to the high thermalconductivity, the heat from the plasma is transferred to the coolantflowing along channels 78 and 79. Anode 1 and the intermediateelectrodes intensively cool the periphery of the plasma, thus forming alarge temperature gradient. Insulator element 17 located in extensionchannel 18, is preferably made of quartz or a ceramic material that hasa very low thermal conductivity. Accordingly, when the heated plasmacomes in contact with insulator element 17 that does not cool plasma,the heat is not distributed through the entire volume of insulatorelement 17. Only the inside surface of the insulator element 17 thatcomes in contact with the heated plasma is rapidly heated to thetemperature of the plasma and is not cooled down. Because there isminimal heat dissipation to the extension nozzle, the temperature of theplasma flowing along the periphery increases. As the heat is transferredfrom the center of the flow to the periphery, the heat is nottransferred to the structural elements of the device. Also, in extensionchannel 18 the center of the plasma flow is not heated by the electricarc, which terminates at the anode. Accordingly, when the plasma flowexits extension channel outlet 55, it has a substantially uniformtemperature and energy density distribution as shown in FIG. 11A. FIG.11B shows that the substantially uniform temperature and energy densitydistribution results in the substantially uniform effect by the plasmaon the treated tissue.

The second, optional, effect of the extension nozzle is generating ozoneand nitric oxide. In some countries it has been recognized that ozoneexhibits properties useful in medicine such as, for example, anantibacterial effect. In other countries, however, the benefits of ozonehave not been recognized. It is well known in the art, however, thatozone may be formed from oxygen by electrical discharges, hightemperature, and exposure to high energy electromagnetic radiation. WhenO₂ molecules are introduced in the plasma flow, some of them aredisassociated into oxygen atoms under the influence of one or more ofthe above conditions, and then recombine with O₂ molecules to form ozone(O₃).

In some embodiments, the device generates ozone, while in otherembodiments, the device does not generate ozone. Generation of ozone maybe controlled in two ways. First, the inlet of oxygen carrying gas maybe controlled by reducing the diameter, or even completely eliminatingoxygen carrying gas inlets. FIG. 12A illustrates extension nozzle 15with inlets 16 of relatively small diameter. FIG. 12B illustratesextension nozzle 15 having no oxygen carrying inlets at all. Second, thelength of the extension channel 18 can be reduced so that oxygenentering through the inlets does not have time to undergo the reactionsrequired to generate ozone. It should be understood that by using one orboth of these ways of controlling generation of ozone, the amount ofozone generated by device 200 may be increased, reduced, or evencompletely eliminated. Similarly, generation of nitric oxide iscontrolled the same ways. The following discussion related to generationof ozone and nitric oxide presumes that extension nozzle 15 has one ormore oxygen and nitrogen carrying gas inlets 16.

Turning to the processes that result in generation of ozone, the plasmaflow, after having traversed the plasma channel, enters extension nozzle15. The temperature of the plasma flow in the extension channel dropspreferably to 3-12° kK. During operation, as the plasma flows by oxygencarrying gas inlets 16, it creates a suction effect in those inlets 16,which results in an oxygen carrying gas, such as air, being pulled intoextension channel 18. In the extension channel, the fraction of air ispreferably in the range of 5-25%, by volume. It is well known that aircontains approximately 21% of O₂ oxygen by volume, and therefore, thefraction of O₂ in the extension channel is preferably in the range of1-5%, by volume. Some oxygen molecules will disassociate into atoms andthen recombine with O₂ oxygen molecules, or sometimes with otherdisassociated oxygen atoms, to form ozone under the influence of twofactors: (1) impacts of O₂ molecules with electrons that have arelatively high energy and (2) the ultra-violet radiation from theplasma channel due to the emission of plasma generating gas molecules,electrons, and other particles. The formation of the ozone moleculesoccurs in accordance with the following chemical reactions:

e+O₂→O+O⁻;

e+O₂→O+O+e; and

O+O₂+M→O₃+M,

where M may be any reacting particle, such as a molecule of a noble gas,for example argon.

Another effect of introducing an oxygen and nitrogen carrying gas intothe plasma flow is generation nitric oxide (NO) in extension channel 18.Various therapeutic effects of NO and methods of its generation are wellknown in the art and are recognized in some countries. For example, U.S.Pat. No. 5,396,882 discloses systems and methods for producing NO byintroducing air into electric arc chamber. Embodiments of the devicehaving an expansion module, likewise, create conditions for producingNO. Introducing a nitrogen and oxygen carrying gas, such as air, intothe plasma flow creates optimal conditions for the synthesis of NO inthe expansion channel 18. As was mentioned above the temperature of theplasma at the anode outlet is in the range of 3°-12° kK. Thistemperature is sufficiently high for the following chemical reaction tooccur in the plasma flow having air molecules, concurrently with theozone production:

N₂+O₂→2NO−180.9 kJ.

In some embodiments, the fraction of air, oxygen or both may be varied.For example, in some embodiments, air enriched with oxygen may besupplied to the oxygen carrying gas inlets 16. In other embodiments, airsupplied to the oxygen carrying gas inlets 16 may be pressurized, thusresulting in a higher concentration of air in the plasma. In yet someother embodiments the combination of the two above methods may be used.

In addition to outputting plasma, and in some embodiments ozone andnitric oxide, the device also emits light due to the radiation from thehigh temperature plasma in the heating portion of the plasma channel. Ithas been discovered and disclosed in, for example, U.S. Pub. No.2003004556 that a pulsed light having a dominant emissive wavelengthfrom about 300 nm to about 1600 nm, where the duration of pulses rangebetween 1 femtosecond to 100 seconds has various therapeutic effects.Among others, treatment of hair, epidermis, sub-surface blood vessels,and many other organs has been shown beneficial with such pulsed light.U.S. Pub. No. 2003004556 discloses various devices and methods forproducing the pulsed light with required characteristics.

As was mentioned above, the temperature of plasma in the heating portionis preferably between 8-18° kK. In this temperature range, the plasmaflow emits light having a dominant emissive wavelength from about 400 nmto about 850 nm. FIG. 13 shows spectral distribution of light 302 at thedistance of about 3 mm from outlet 55 and spectral distribution of light304 at the distance of about 50 mm from the outlet 55. FIG. 13 shows thefollowing spectral distribution:

200-350 nm—2%;

350-400 nm—5%;

400-650 nm—62%;

650-750 nm—15%;

750-850 nm—14%; and

850-1400 nm—2%.

Accordingly, device 200 may be used for pulsed light therapy inconjunction with its other uses. Note that the ratio of shorterwavelengths to longer wavelength in the spectrum of the emitted lightmay be easily changed by adjusting the magnitude of the current passingthrough the plasma flow during operational periods of pulses. Withincreased current, approximately the same amount of energy is used forplasma generation, but substantially more energy is used for lightemission.

As for treating patients, the device may be used safely and effectivelywithout the need to remove it from the treated tissue after each pulse,as has to be done with some prior art devices. Therefore, pulses ofplasma may be generated automatically with relatively high frequency.For each pulse, a new plasma flow is generated by first passing throughspark discharge and glow discharge phases, and then heating the plasmagenerating gas with an electric arc during the arc discharge phase. Oncethe plasma flow is established, it is expanded in the plasma channel bypassing through the sections of the expansion portion, then the anodeportion, and then the extension channel. In the extension channel, thethermal and energy density distribution of the plasma flow is modifiedto be substantially uniform across the cross section of the extensionchannel, as described above. The expanded plasma flow with the modifiedthermal energy distribution is safely applied to the patient's skin forthe duration of the pulse. At the end of the pulse the plasma flowceases entirely. This process can be repeated until the desired numberof pulses has been delivered. The light radiation which is generated mayprovide benefits for treating the skin and sub-surface organs, such asdermis and blood vessels, in addition to the benefits resulting from theplasma pulses.

Extraneous matter is removed from the surface of the treated skin.Removal of extraneous matter does not have to be synchronized withpulses and may be a continuous operation. Additionally, ozone may bemixed into the plasma flow applied to the patient's skin for additionalbeneficial effects. As discussed above, introducing an oxygen carryinggas in the inlets of the extension portion results in formation of ozonemolecules in the plasma flow.

Importantly, after a pulse of plasma is applied to the skin, the plasmaflow ceases completely until the next pulse. During the off period,plasma is not applied to the patient's skin and the patient is affectedonly by the harmless flow of a cool plasma generating gas and the vacuumsuction of the extraneous matter pump. Accordingly, an operator usingthe device does not risk errors associated with removal of the devicefrom a patient's skin during the off-period and then attempting tocorrectly reposition the device to continue treatment. Thissubstantially improves the safety and the duration of the procedure.

The foregoing description of the embodiments of the present inventionhas been presented for purposes of illustration and description. It isnot intended to be exhaustive nor to limit the invention to the preciseform disclosed. Many modifications and variations will be apparent tothose skilled in the art. The embodiments were chosen and described inorder to best explain the principles of the invention and its practicalapplications, thereby enabling others skilled in the art to understandthe invention. Various embodiments and modifications that are suited toa particular use are contemplated. It is intended that the scope of theinvention be defined by the accompanying claims and their equivalents.

1. A method of generating pulses of plasma using a device comprising acathode and an anode, the method comprising: a. passing aplasma-generating gas between the cathode and the anode; and repeatedly:b. establishing an electric arc between the cathode and the anode; c.controlling an area of attachment of the electric arc to the cathode;and d. terminating the electric arc.
 2. The method of claim 1 furthercomprising generating a plasma flow while the electric arc isestablished between the cathode and the anode.
 3. The method of claim 2,wherein the plasma flow is substantially free of impurities.
 4. Themethod of claim 1, wherein controlling the area of attachment of theelectric arc to the cathode is accomplished by setting the current inthe electric arc to a predetermined value.
 5. The method of claim 4further comprising heating the plasma flow after controlling the area ofattachment of the electric arc to the cathode.
 6. The method of claim 1,wherein controlling the area of attachment of the electric arc to thecathode comprises reducing a size of the attachment of the electric arcto the cathode by reducing the current in the arc.
 7. The method ofclaim 2, further comprising controlling a thermal distribution of theplasma flow by passing the plasma flow along a surface with low thermalconductivity.
 8. The method of claim 7, wherein the thermal distributionof the plasma flow, after passing the surface with low thermalconductivity, is substantially uniform in a cross-section.
 9. A methodof generating nitric oxide using a device, the method comprising: a.passing a plasma flow along an electric arc; and b. subsequently,introducing into the plasma flow a gas comprising nitrogen and oxygenthrough a channel in the device.
 10. The method of claim 9 furthercomprising synthesizing nitric oxide in the device.
 11. The method ofclaim 10, wherein the plasma has a temperature in the range 3-12 kK inthe area where the gas comprising nitrogen and oxygen is introduced intothe plasma flow.
 12. The method of claim 10, wherein the gas comprisingnitrogen and oxygen is introduced under pressure that exceeds theatmospheric pressure.
 13. The method of claim 10, wherein the plasmaflow emits a light.
 14. The method of claim 13 wherein the light has adominant emissive wavelength of 400 nm-850 nm.
 15. A plasma-generatingdevice comprising: a. an anode; b. a cathode assembly comprising (i) oneor more cathodes, and (ii) a cathode holder; c. a plasma channelextending longitudinally between said cathode assembly and through saidanode, the plasma channel having an outlet opening at the anode end; d.an extension nozzle connected to the anode end of the plasma channel andforming an extension channel.
 16. The plasma-generating device of claim15, wherein the plasma channel is expanding toward the anode.
 17. Theplasma-generating device of claim 15 further comprising a plasmachamber.
 18. The plasma-generating device of claim 15, wherein theextension nozzle has a tubular insulator covering a portion of theinside surface of the extension channel.
 19. The plasma-generatingdevice of claim 15, wherein the extension nozzle has one or more gasinlets to the extension channel.
 20. The plasma-generating device ofclaim 15 further comprising a cooling channel capable of cooling theanode.